Thermosetting resins having a benzoxazine ring in their molecular structure have excellent properties which are not observed in other thermosetting resins, such as dimensional stability, electric insulation properties and low water absorption properties in addition to heat resistance and flame retardancy. Therefore, it has recently drawn attention as an electronic material such as a laminate sheet and a semiconductor sealing material and a binder such as a friction material and a grinding stone.
A benzoxazine ring is a composite ring of a benzene ring and an oxazine ring and has a structure represented by the following formula (I):
(In formula (I), R1 represents any organic group.)
In order to efficiently synthesize a thermosetting resin having a benzoxazine ring which has the excellent properties as described above, technological studies have been made on a synthetic method thereof in recent years. The technical points for efficiently synthesizing the same are as follows: 1) prevention of gelation of a reaction solution and insolubilization of a reaction product by controlling the reaction; and 2) facilitation of a synthetic process.
Patent Document 1 discloses a method in which phenols, paraformaldehyde, and monoamines are reacted in an organic solvent, specifically, an alcoholic solvent.
However, the method disclosed in Patent Document 1 employs a process of adding firstly phenols and paraformaldehyde to an organic solvent and controlling the temperature and then adding monoamines little by little, in order to suppress a rapid progress of the reaction due to a heat generation during the synthesis and the gelation of the reaction solution and insolubilization of the reaction product accompanying the rapid progress of the reaction. Therefore, equipments such as a temperature controller and a dropping pump are required, and the resulting synthetic process is complicated. It is also necessary to pay great attention to the temperature control during the synthesis.
Patent Documents 2 and 3 disclose methods of using an aprotic solvent (except dioxane) as an organic solvent in order to solve the above problems.    [Patent Document 1] Japanese Patent Laid-Open No. 2000-273135    [Patent Document 2] Japanese Patent Laid-Open No. 2002-338648    [Patent Document 3] Japanese Patent Laid-Open No. 2005-213301